The present invention relates to a butterfly valve assembly for a fuel injection apparatus of an internal combustion engine, in particular for an injection apparatus that includes electronic means for controlling the quantity of fuel injected during each operating cycle of the engine.
The quantity of fuel injected per cycle must be proportional to the flow rate of air admitted to the engine. In many injection devices used at present, the air flow rate is computed by electronic means on the basis of signals that indicate firstly the extent to which the butterfly valve member is open (e.g. as provided by a potentiometer), and secondly the pressure differential between the upstream side and the downstream side of the valve, which difference is representative of the air speed. Satisfactory control of the engine at low loads (i.e. when the degree of butterfly valve opening is small) requires that any initial increase in flow rate take place very smoothly. In particular, when using an electronically controlled injector, drivability can be obtained in practice only when the initial change in air flow rate as a function of the opening angle a of the butterfly valve has a shape of the kind shown diagrammatically by the heavy line in FIG. 1.
To obtain the required smoothness, a butterfly valve assembly has already been proposed of the type that includes a body formed with an admission duct or passage and a butterfly valve member in the form of a disk that is circular or slightly elliptical, the disk being mounted on a middle rotation shaft that extends transversely to the passage with the valve member being displacable between a position of minimum opening (possibly zero opening) and a position of maximum opening in which the butterfly valve member lies substantially parallel to the axis of the duct, the duct having a wall that is complex in shape such that at the beginning of opening, the cross-sectional air flow area varies as a function of the opening angle of the butterfly valve member much less quickly than would be the case in a cylindrical duct.
If the body is made of metal, where molding does not give any good degree of finish, the complex shapes used heretofore cannot be achieved by simple machining and/or require blanks that are difficult to cast. Furthermore, many shapes do not make it possible to assemble the butterfly valve member merely by inserting it along the axis of the duct, while it is a condition which is essential on an automatic assembly line.
Proposals have nevertheless been made (French patent application FR 90 07848) for a butterfly valve body having a duct of complex shape that can be achieved directly by casting, but only if made of a synthetic material where casting gives sufficient accuracy. However, the shape proposed in application No. 90 07848 is based on an approach that takes into consideration cross-sections rather than flow rates, and as a result it does not make it easy to satisfy all of the relationships between degree of opening and flow rate that are required by present-day manufacturers, which relationships differ considerably depending on the engine being fed or the construction of the driving linkage.